This invention relates to the removal of excess molten aluminum and molten aluminum alloys from metal articles aluminized by the hot-dip method.
The aluminum coating produced by the hot-dip method consists of two layers: an outer layer and a transitional layer. The outer layer, which contains areas of almost pure aluminum has a decisive influence upon the corrosion properties of the coatings, especially in oxidizing environments, both in ambient and high temperatures, caused by the formation of a thin Al.sub.2 O.sub.3 layer on the coating surface.
The outer layer is formed of an FeAl.sub.3 phase in the form of columnar crystallites which are its frame during the aluminizing process. The free spaces between the crystallites are filled with an aluminum rich composition comprising 98% Al/eutectic composition (Al and FeAl.sub.3).
The transitional (intermetallic) layer is characterized by particular corrosion resistance in brines and in lyes and in some acid solutions. Mainly, that layer consists of two zones, which beginning from the surface create: a thin layer of minute FeAl.sub.3 crystals and a considerably thicker layer of directionally growing columnar cristallites Fe.sub.2 Al.sub.5 phase whose cross section is visible as "teeth" penetrating into the substrate.
The thickness of the outer aluminum layer is most important for preserving the primary shape of the original product. Complex shaped elements, such as for example, fine pitch threads when removed from the aluminum coating bath are frequently considerably distorted with the grooves or indentations all filled in with a molten outer layer of aluminum.
In the methods of the prior art, the removal of the liquid outer layer is accomplished by reducing the viscosity of the molten aluminum or aluminum alloys. This is obtained either by increasing the temperature of the molten bath or alloying with metals, such as lithium, sodium, or beryllium. Employing low speeds when the article is withdrawn from the molten bath is also significant in removal of excess molten aluminum.
One of the well known methods for the removal of excess aluminum and its alloys from the surfaces of compact shaped elements, such as white malleable cast iron bodies consists in withdrawing such a product from the molten bath at a linear speed of 0.5 m per minute from a bath containing 4% silicon at a temperature of 750.degree. C. and subsequently slowly cooling the products in the atmosphere.
These known methods for removal of excess molten aluminum and its alloys from the surface of aluminum coated articles have many disadvantages since they are deficient in removing excess aluminum and its alloys from complex shaped articles particularly from the grooves and holes of threaded elements.
Accordingly the traditional hot-dip method for coating aluminum can be employed only for such types of simply shaped articles as wire or steel tape and elements of compact shape having no incisions, holes or indentations. Such articles are most often simply shaped castings and forgings.
The traditional hot-dip method cannot be used for aluminum coating of detailed articles or those of a small size which previously have been finished by machining, abrading or filing to an exact shape, dimension or close tolerances, as is the case with finely threaded elements. Excessive aluminum or its alloys on the surface of coated articles form the shape of icicles and drops which clearly deform the coated article and consume excess quantities of aluminum and its alloys, thus increasing the cost of the process. Finally, the excess aluminum on the surface of the coated articles makes for an uneven coating layer.
In view of the fact that the thickness of the outer layer depends largely on the bath temperatures and the immersion time, simultaneous thermal treatment and coating of the articles with aluminum can be very difficult, or even impossible, because during longer heat treatment of articles immersed in an aluminum bath, for example 25 minutes, the outer layer would be so thick that it would distort the original shape of the article. Additionally, the high-temperature bath increases the cost of the process. The efficiency is also limited by the low speeds of withdrawal from the bath used in the known method.
The novel method of this invention obviates these disadvantages with its procedure detailed in the following.